National elementary science standards have included the teaching of systems for decades, however little is known about teachers' understanding of complex systems. This study focuses on how elementary pre- and in-service teachers understand and apply "systems thinking" to the water cycle. Teachers completed the Water Cycle Diagnosis Test and the Systems Thinking Assessment. Results showed that more in-service teachers (54.1%) used high-level systems thinking explanations than pre-service teachers (33.3%). Although neither group of teachers scored at the intermediate level (highest level) of systems thinking. Common barriers were identified with both teacher groups, which included identifying components and processes, recognizing multiple interactions and relationships between subsystems and hidden dimensions, and finally, difficulty understanding the human impact on the water cycle system.